A "butt" was a medieval unit of measure for wine or beer. Technically a butt load is 475 liters or 126 gallons, which is roughly the size of the biggest wine and beer barrels from medieval Europe.
No worries, I've ordered a buttload of beer for the party, so we'll be good.
n. The amount that can be loaded into the buttocks while causing minimal discomfort. This measurement is not constant but varies depending on the individual. Related factors include rectal diameter, anal elasticity, and cheek-to-hole ratio.
Susie's rear was in pain the next day after taking a full butt-load the previous night.